Development of mechanistic models for the description of radionuclide retention onto calcareous gravels

Abstract

The retention capacity of natural calcareous gravels used as a filling material between the packages containing radioactive waste materials in the disposal cells was evaluated for Sr, U, Ni, Ag, ¹⁴C, ⁹⁹Tc, ²³⁹Pu, ²²⁸Th and ¹⁵²Eu. The thermodynamic calculations performed considering repository conditions indicated that the chemistry of most of these elements was dominated by neutral or cationic species, this is the case of ²³⁸Pu ¹⁵²Eu, ²²⁸Th, Ni and Ag. The studied gravels presented high sorption capacities for these cationic or neutral species, but they are not efficient on retaining anionic species as in the case of ⁹⁹Tc or Sr, which is already present in the composition of the studied gravels. For those elements where the predominant species are carbonated (¹⁴C and U) low distribution coefficients were obtained. A preliminary mechanistic sorption model was developed for each radionuclide considering carbonate (>CO₃H) and calcium hydroxide (>CaOH) sites. Our preliminary model allowed to successfully reproduce the experimental trend of the data obtained in this work.

Keywords: Actinides; Calcite; Modelling; Radionuclide; Sorption.